Light-sensitive control systems are known which usually rely on a photoresistor, photodiode, or photocell which produces a resistance, current, or voltage difference proportional to the intensity of light striking its light-sensitive surface. This output is fed to a relay so that when the light level crosses a predetermined threshold the relay opens or closes to operate the motor that displaces the jalousies, roller blinds, or the like. As a general rule a time delay of at least two minutes is provided between the trigger and the relay so that temporary weather or environmental changes do not actuate the load or cause the device to operate too frequently and consequently to wear out rapidly.
The main problem with such devices is in distinguishing between the diffuse ground light which is reflected from buildings, the ground, and the like, and the parallel rays of sunglight which originate directly from the sun. The ground light changes according to season, time of day, cloudiness, sizes and shapes of surroundning structures, and the reflectivity of these neighboring structures as well as of any ground cover such as snow or water. The sunlight intensity alone should be determinative when jalousies or the like are to be set. It is common to have ground or background light which rises to a level of 30,000 lux, whereas morning sun, for instance, might only have a level of 3,000 lux. This latter light level should actuate the control, whereas the higher ground-light level should not. Thus a ground light suppression factor (d.sub.0) of ##EQU1## IS NECESSARY IF THE CONTROL IS TO TRIP WITH WEAK MORNING LIGHT BUT NOT WITH BRIGHT MIDDAY GROUND LIGHT.
The sunlight irradiates the photosensitive surface of the control through a window which is open both horizontally (aperture angle .alpha.) and vertically (aperture angle .beta.). Clearly the light intensity on a surface is proportional to the angle of incidence of the rays upon the surface; therefore this intensity is proportional to cos .alpha.. cos .beta.. This should be clear from the fact that a light source lying directly above the surface will throw the most light on a surface (cos 90.degree. = 1.0) and a source whose rays are parallel to the surface will not irradiate it at all (cos 0.degree. = 0.0). This gives a necessary ground light suppression factor d.sub.nec equal to d.sub.0 /(cos.alpha..cos.beta.), or 10/(cos.alpha..cos.beta.).
In conventional systems it is difficult to control the threshold of the apparatus, especially in view of the above-described difficulty with suppression of ground light. The common solution has been to provide the light-sensitive element deep within an upright tube, so that only light from the sky can fall on it. In order to collect sufficient light it is known in these arrangements to use a collecting lens. In this manner the horizontal and vertical angles, .alpha. and .beta., respectively, are limited. The collimating tube is fixed so that it is directed at the sky, usually generally to the south when used in the Northern Hemisphere. As a rule the angle .alpha. must be equal to at least 150.degree. and angles.beta. must be equal to at least 80.degree., so that the entire arrangement must be fairly complicated and expensive. Even the best of such devices operates with a ground-light suppression which is often far from satisfactory and requires periodic readjustment to compensate for seasonal light changes.